1. Field of the Invention
This invention relates to an ultrasonic Doppler diagnostic device, more particularly to an improved ultrasonic Doppler diagnostic device for detecting blood flow within a living organism or for measuring the velocity or the amount of such blood flow.
2. Description of the Prior Art
The ultrasonic Doppler method is widely used for noninvasively detecting and measuring the movement of a moving member within a living organism and, specifically, is commonly used for noninvasive detection and measurement of blood flow within the heart, blood vessels etc.
Generally speaking, however, the ultrasonic wave reflected from the blood flow is mixed together with an ultrasonic wave component reflected by the wall of the blood vessel or the heart which surrounds the blood flow (such component being referred to as the "wall signal" hereinafter). This is particularly troublesome since the the blood flow signal is much weaker than the wall signal.
Although the amplitude of the wall signal can be reduced by focusing the ultrasonic wave into a fine beam, there is a limit to how sharply the beam can be focused so that even when this method is used the reflected wave nevertheless includes a high-level wall signal along with a low-level blood flow signal. As a result, there has been the disadvantage that the strong wall signal makes it difficult to obtain the desired blood flow signal.
Conventionally, the separation of the blood flow signal from the wall signal has been carried out on the basis of frequency difference. This is possible since the wall signal is a wave reflected from a substantially nonmoving portion so that its frequency following comparison with the reference wave according to the Doppler method is relatively low, whereas the corresponding frequency for the wave reflected from the moving blood is, because of the Doppler effect, relatively high. This frequency difference can be used to separate the two signals. More specifically, by passing the mixed signals through a filter, the strong but low frequency wall signal can be separated from the weaker, higher frequency blood flow signal.
Nevertheless, conventional devices operating on this principle have not been able to produce the desired results when the detection or measurement is carried out for blood flow within very fine blood vessels. This is because in such cases the ratio of the wall signal to the blood flow signal becomes extremely large so that when the blood flow signal is amplified by a degree adequate to make it detectable, the wall signal, which is amplified by the same degree, becomes so large as to saturate the receiver circuit. As a consequence, it becomes difficult, if not impossible, to detect the blood flow signal.